The U.S. Navy's top "spy catcher" is killed by a powerful letter bomb while in Honolulu. Five-O's Steve McGarrett, while serving a two-week hitch in the Navy Reserves, is called upon to investigate the slaying. The death is tied to a security leak within Naval Intelligence and it turns out that Wo Fat is its mastermind.

Mobster Charlie Bombay, an old foe of McGarrett's, escapes from prison, thanks to a bribed guard. But Bombay's confederates tip Five-O to Bombay's whereabouts before Charlie can flee Hawaii. McGarrett arrests Bombay in Hilo, thanks to the tip and wants to take the criminal back to prison personally. But the small police airplane containing McGarrett and Bombay crashes because of turbulent weather. The pilot is killed in the crash and Bombay overpowers an injured McGarrett.

A British Intelligence agent helps McGarrett try to find a missing nobleman who is suspected of treason and murder.

Susan has been having an affair with a man involved in skimming the gambling take from some Las Vegas casinos and has been acting as a courier, taking the money to Hawaii. She decides to get out and, in a fight, with her boyfriend, shoots him. She thinks she killed him. What she doesn't know is that a hit man entered the boyfriend's apartment minutes later and killed the man. Now, Susan is on the run -- not knowing the hitman is about to make her his next target. Five-O tries to find Susan before it's too late.

A man arrives at Honolulu International Airport and is taken to the hospital after being struck by a car. Upon reviving, he checks the bag he was carrying and, being unable to find something in it, takes a cyanide capsule he had hidden in his jacket pocket and dies. The hospital had taken the object and put in a safe with his other valuables. Five-O discovers the object is part of a small surface-to-air missile. McGarrett recruits a police officer who can pass for the man and he will make attempt to make contact with those the dead man was going to meet. Five-O uncovers a plot to assassinate an exiled political leader who is heading to Hawaii for a layover as part of a trip to his homeland. The conspirators soon use one of their SAMs to avoid detection. The second will be targeted at the exiled political leader and everyone who is on the same commercial flight he is.

After being denied parole from prison, crime lord Honore Vashon comes up with a twisted scheme to exact revenge on McGarrett for the deaths of his son Chris and his father Dominick by taking a group of prison officials hostage. He then exchanges the officials for McGarrett and then places McGarrett on a mock trial for the alleged crimes he committed against the Vaschon family. Can Danny and the rest of the Five-O team save Steve before Vaschon sentences McGarrett?

A spy ring targets a U.S. missile project headed by Dr. Grant Ormsbee, a headstrong scientist. The bait is an agent posing as an Asian scientist Orsmbee had met 12 years previously and who had maintained correspondence with Ormsbee. McGarrett clashes with Ormsbee initially when the scientist refuses to cooperate with Five-O's investigation of the death of a member of the spy ring, who has been sacrificed as part of the group's complicated plot. Ormsbee eventually agrees to play Judas Goat himself as Five-O and Naval Intelligence attempt to smash the spy ring.

Singer Chelsea Merriman witnesses Koko Apaleka striking his girlfriend, causing her to fall from a high-rise balcony. Chelsea initially tries not to become involved, resisting McGarrett's attempts to get her to testify. But, after Koko tries to intimidate Chelsea by sending a doll with a broken head to Chelsea's daughter on the Mainland, the singer decides to cooperate with the authorities. Now, McGarrett & Co. must protect Chelsea from Koko's attempts to have her killed.

Dan Williams welcomes his Aunt Clara to Hawaii for a visit. On the flight over, she got to know Edgar Miller, an elderly man on the same flight. Once he checks into his hotel room, Miller encounters two attackers but not before he makes a telephone call to Clara. Five-O's Duke, investigating Clara's call, is initially taken in by an impostor pretending to be Miller. But Five-O, after the real Miller's body is discovered, begins an investigation. McGarrett & Co. utilize Clara as part of a con to bring the conspirators to justice.

Morwood, a high school official, has organized a smuggling ring. His confederates are forced to toss a drug shipment overboard as the Coast Guard is ready to close in. Morwood's group knows the approximate position but can't make a move because Five-O and the authorities have the heat on. But when Morwood spots a mini-submarine he gets the idea the the vessel could snatch the drugs out from under the nose of the police.

Mendoza is the manager of a string of warehouses along Hawaii's waterfront. He, his daughter and a group of hired thugs knock over the warehouses one by one and steal their contents. A robbery goes horribly wrong and ends in the death of an HPD officer. A robber, shot in the stomach, manages to get away with the manager's daughter in tow. They park their truck in a deserted area and try to hide. What they don't know is that their stolen cargo is a vat of highly volatile chemicals which will explode under the tropical sun in a matter of hours.

Egotistical author Travis Marshall discovers the unmarked grave of heir Brian Henderson. Marshall is promoting his find to the media, much to the annoyance of McGarrett & Co., who have re-opened an investigation. Brian Marshall has been missing for seven years. Five-O has to reconstruct what happened to the heir, with no help from Marshall or Agatha Henderson, the wealthy grandmother of the dead man. The case turns out to be an elaborate jigsaw puzzle for McGarrett, which is further complicated when Marshall turns up dead.

Three college football players, one the son of a powerful senator, rape a waitress. The senator sends his "fixer," a lawyer working for him, to take care of the situation. The fixer finds a petty criminal willing to be a patsy in return for $5,000 and a guarantee that charges won't be pressed against him. Five-O tries to find out what really happened. Meanwhile, the victim moves to take matters into her own hands.

August March, a seemingly respectable businessman who is also an art collector, has been running a smuggling ring of Asian art. When he discovers McGarrett is also a collector of small Asian sculptures, March arranges for a stolen piece of art from Japan to be substituted for McGarrett's collection, which is about to go on display in a Honolulu museum. March sees this an opportunity to frame the lawman who could smash his operation. But McGarrett, despite all odds, moves to trap his opponent.

Deadly liquefied nerve gas is stolen. Five-O investigates the theft; the substance can kill a person if only a drop touches their skin and a small quantity can cause mass casualties. But the thief isn't a terrorist or master criminal, he's a young man seeking revenge against Dan Williams. Danno investigated the young man's father, a corrupt police officer. The young man concocts a frame for Williams that Five-O will find difficult to prove false.

Alex Kelsey, a lawyer involved with mobster Din Lee, kills a dying Japanese man. The dead man had been a Japanese operative at the time of Pearl Harbor. Kelsey believes the dead man had a secret stash of gold. Kazuo Tahashi, the dead man's son, arrives from San Francisco. But Tahashi isn't all what he appears to be, either. Meanwhile, in the background is retired U.S. Navy Commander Reginald Blackwell, who had been an operative in Naval intelligence. McGarrett & Co. must try to put the pieces of this jigsaw puzzle together.

Five-O is protecting Billy Madrid, lieutenant to a crime boss who will testify in an upcoming trial. McGarrett is wounded when a sniper shoots at Five-O officers escorting Madrid, an indication that the crime boss wants Madrid dead. But when the trial actually starts, Madrid tells a different story -- that McGarrett forced his testimony. Now, McGarrett & Co. must find a way to recoup or else the crime boss will go free.

Five-O begins an investigation after it's discovered a new trade center, destroyed in a massive fire, was constructed of sub-standard materials. The probe centers on Maynard, who works in the local agency that inspects building plans and construction sites. Maynard has framed, and then kills, one inspector. Five-O attempts a sting operation with the cooperation of one building but that plan goes awry when Maynard realizes he is being recorded while soliciting a bribe.

While flying her brother's hang glider over Makapuu, Molly Taggart witnesses a friend's murder by a pair of local low life thugs. Realizing that their crime was seen from the air, the thugs set out to kill the glider's pilot.

An Australian scientist is killed on a Hawaiian sugar plantation after discovering an insect capable of wiping out the sugar cane there. The investigation leads McGarrett and Co. to a complicated plot hatched by crooked developer Sam Patton (portrayed by Richard Kiley) to buy the plantation for a greatly reduced price. Before Five-O can solve the case, the death toll will rise.

Two idealists plan to kidnap the son of a Japanese industrialist as a way to generate $1.5 million toward their dream of researching ways for man to live under the sea. They don't realize their confederate is really part of a Japanese terrorist group. She has no intention of letting the boy live. Five-O has to race against time. The kidnapped child is in an airtight capsule underneath the ocean and he has only a limited air supply.

A rich developer's wife has been kidnapped and an ecology activist has been implicated. But the activist is innocent and has been framed. McGarrett & Co. try to solve the crime and find the kidnapped woman. As the title implies, the developer's neighbor holds the key to the mystery.

A series of gold robberies has hit Oahu. The operation is being run out of a halfway home for boys and young men. McGarrett sends in an HPD undercover officer to the home to turn up some leads. The trail first seems to point to the home's administrator, an ex-con. He turns out to be innocent. But Five-O's investigation is pressuring the organizations of the robbery ring.